Page 269 - Practical Ship Design

Powering I1 227

Fig. 7.17. Mitsui duct. Fig. 7.18. Hull vanes - Grothues spoilers.

Although it took naval architects a long time to think of this idea, or at any rate to
use it in practice, it seems only sensible to shape the ship ahead of a propeller
designed to rotate in one direction in a way that takes this into account.
An alternative to asymmetrical lines is to move the propeller a little off centre
(to starboard for normal clockwise rotation) when the water flow reaching it will
be affected in much the same way as it is by asymmetrical lines.
Other devices, all of which aim to induce a more uniform flow into the
propeller, include the Mitsui duct shown in Fig. 7.17, the Schneekluth wake
distribution duct and Grothues spoilers (Fig. 7.18). All of these devices necessarily
add some extra resistance as well as cost and weight. In addition to any improve-
ment in propulsive efficiency, these devices can reduce propeller excited vibration
forces and have been used in retrofits for this purpose.
The action of these devices is of course much more local than that of asym-
metrical lines and there seems no reason why one of them should not be combined
with asymmetrical lines.

7.8.2 Post propeller recovery

An alternative to avoiding or reducing rotational energy loss in the way just
mentioned is to seek to reclaim some of the rotational energy from the water abaft
the propeller and this is the principle of the Grim wheel. A Grim wheel, shown in
Fig. 7.19, has a larger diameter than that of the propeller in whose wake it is placed
so that it can also reclaim some tip vortex energy. There seems little doubt that this
device can be effective in its energy reclaim role, but the break-up of some wheels
poses the question of whether the forces involved are fully understood.